Method and apparatus for transmitting signals over wire connections

ABSTRACT

A method and apparatus for transmitting data from a transmitter device to one or more receiver devices connected to the transmitter device via a respective wire connection, the transmitter device being operable to transmit signals onto the wire connections and a further wire connection at different tones, the method comprising: for each tone, measuring electromagnetic coupling between the further wire connection and each of the wire connections and using the measurements, identifying a wire connection that most strongly receives crosstalk interference from the further wire connection; based on the identifications, for each tones, allocating signals transmitted on the further wire connection as supporting signals for a particular wire connection; transmitting a first signal onto the particular wire connection that has been allocated a supporting signal; and transmitting a second signal onto the further wire connection, thereby to cause crosstalk interference in the particular wire connection transmitting the first signal.

This application is the U.S. national phase of International ApplicationNo. PCT/EP2018/058500 filed 3 Apr. 2018 which designated the U.S. andclaims priority to EP Patent Application No. 17164439.6 filed 31 Mar.2017, and EP Patent Application No. 17173054.2 filed 26 May 2017, theentire contents of each of which are hereby incorporated by reference.

FIELD OF INVENTION

The present invention relates to a method and apparatus for transmittingdata from a transmitter device to a plurality of receiver devices, andin particular to a method and apparatus for transmitting and receivingdata signals over pairs of wires. Such methods include all of thevarious Digital Subscriber Line (DSL) methods as specified in variousInternational Telecommunications Union (ITU) standards and as beingfurther developed in the ITU at present. Typically each such pair ofwires comprises a twisted metallic pair (usually copper) as commonlyfound within telephone access networks throughout the world.

BACKGROUND

DSL technology takes advantage of the fact that although a legacytwisted metallic pair (which was originally installed to provide merelya Plain Old Telephone Services (POTS) telephony connection) might onlyhave been intended to carry signals using differential mode atfrequencies of up to a few Kilohertz, in fact such a line can oftenreliably carry signals at much greater frequencies. Moreover, theshorter the line, the greater is the range of frequencies over whichsignals can be reliably transmitted (especially with the use oftechnologies such as Discrete Multi-Tone (DMT), etc.). Thus as accessnetworks have evolved, telecommunications network providers haveexpanded their fibre optic infrastructure outwards towards the edges ofthe access network, making the lengths of the final portion of eachconnection to an end user subscriber (which is still typically providedby a metallic twisted pair) shorter and shorter, giving rise tocorrespondingly greater and greater bandwidth potential over theincreasingly short twisted metallic pair connections without having tobear the expense of installing new optic fibre connections to eachsubscriber. However, a problem with using high frequency signals is thata phenomenon known as crosstalk can cause significant interferencereducing the effectiveness of lines to carry high bandwidth signals insituations where there is more than one metallic pair carrying similarhigh frequency signals in close proximity to one another. In simpleterms, the signals from one channel can “leak” onto a nearby channel(which may be carrying similar signals) and appear as noise to thatnearby channel. Although crosstalk is a known problem even at relativelylow frequencies, the magnitude of this effect tends to increase withfrequency to the extent that at frequencies in excess of a few tens ofMegahertz (depending on the length of the channels in question), theindirect coupling can be as great as the direct coupling.

In order to alleviate the problems caused by crosstalk (especially FarEnd Cross Talk or “FEXT” as it is known) a technology known as vectoringhas been developed in which knowledge of the signals sent overcrosstalking lines is used to reduce the effects of the crosstalk. In atypical situation a single DSLAM acts as a co-generator of multipledownstream signals over multiple cross-talking channels and also as aco-receiver of multiple upstream signals from the same multiplecross-talking channels, with each of the channels terminating at asingle Customer Premises Equipment (CPE) modem such that no commonprocessing is possible at the CPE ends of the channels. In such a case,downstream signals are pre-distorted to compensate for the expectedeffects of the cross-talking signals being sent over the neighbouringcross-talking channels such that at reception at the CPE devices thereceived signals are similar to what would have been received had nocross-talking signals been transmitted on the cross-talking channels.Upstream signals on the other hand are post-distorted (or detected in amanner equivalent to their having been post-distorted) after beingreceived at the co-receiver (the DSLAM) in order to account for theeffects of the cross-talk which has leaked into the signals during theirtransmission.

SUMMARY OF INVENTION

The present inventors have realised that, as the bandwidth of DSLsystems keep expanding, the encountered loss will tend to become steeperthan earlier DSL generations. Thus, flat Power Spectral Density (PSD)will tend to become extremely inefficient. This may be due, for example,to non-optimal power allocation to some frequencies. Aspects of thepresent invention provide a fast converging analytical water-fillingalgorithm that tends to overcome this problem.

The present inventors have further realised that, unlike slow/fast timevarying wireless channels, Twisted Metallic Pair (TMP) channels tend tobe semi-static. Thus, variability of the frequency response of TMPchannels over time tends to be negligible. The present inventors havefurther realised that, however, with TMP channels the loss across thesystem bandwidth tends to be much steeper compared to wireless channels,and, as a result, aggressive signal shaping of the channel frequencyresponse for TMP channels can be avoided. The present inventors haverealised that this can be exploited to minimise the complexity of anywater-filling solution.

In a first aspect, the present invention provides a method oftransmitting data from a transmitter device to one or more receiverdevices. Each of the one or more receiver devices is connected to thetransmitter device via a respective wire connection. The transmitterdevice is operable to transmit signals onto the wire connections at oneor more different tones. The transmitter device is further operable totransmit signals onto a further wire connection at the one or moredifferent tones. The method comprises: for each of the one or moretones, measuring, for each of the wire connections, an electromagneticcoupling between the further wire connection and that wire connectionfor that tone; for each of the one or more tones, using theelectromagnetic coupling measurements, identifying a wire connectionthat most strongly receives crosstalk interference from the further wireconnection at that tone; based on the identifications, for each of theone or more tones, allocating signals transmitted on the further wireconnection at that tone as supporting signals for a particular wireconnection; for one or more of the tones: transmitting a first signalonto the particular wire connection that has been allocated a supportingsignal at that tone; and transmitting a second signal onto the furtherwire connection at that tone, thereby to cause crosstalk interference inthe particular wire connection transmitting the first signal.

Measuring the electromagnetic coupling between the further wireconnection and that wire connection may comprise measuring a value of achannel transfer function between the further wire connection and thatwire connection.

In some aspects, the further wire connection is not connected betweenthe transmitter device and the one or more receiver devices.

The method may further comprise determining, for each tone indexed by m,for each wire connection indexed by i, and for the further wireconnection indexed by j, a value of:

$\mu_{i,j}^{m} = {\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)\begin{Bmatrix}{{\log_{2}\left\lbrack \frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)}{\ln\mspace{14mu} 2\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)} \right\rbrack} -} \\\left\lbrack {\frac{1}{\ln\; 2} - \frac{\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)}} \right\rbrack\end{Bmatrix}}$where γ_(i,j) ^(m) is a channel gain in the ith wire connection causedby the further wire connection indexed by j; and

$\Omega_{i,j},\eta_{i,j}^{m},\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m},{{and}\mspace{14mu}\beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}}$are variables. The step of identifying may comprise using the determinedvalues of μ_(i,j) ^(m).

In some aspects,

$\Omega_{i,j} = \frac{M}{\ln\mspace{11mu} 2\left( {\frac{P_{T}}{\Delta_{f}} + {\sum\limits_{m}\frac{1}{\gamma_{i,j}^{m}}}} \right)}$

where M is the number of different tones; Δ_(f) is a frequency spacingbetween adjacent tones; and P_(T) is the maximum power for transmittingthe data.

In some aspects,

$\eta_{i,j}^{m} = {\frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}} \right)}{\left( {\ln\mspace{11mu} 2} \right)\left( {1 + {p_{m}\gamma_{i,j}^{m}}} \right)} - \Omega_{i,j}}$where p_(m) is a power mask at tone m.

In some aspects,

$\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} = {1 - \frac{2^{b_{\max}}\left( {\ln\mspace{11mu} 2} \right)\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}}}$where b_(max) is an upper bound for a channel capacity of the wireconnections.

The method may further comprise determining, for each tone indexed by m,for each wire connection indexed by i, and for the further wireconnection indexed by j, a value of:

$\Gamma_{i,j}^{m} = \frac{\gamma_{i,j}^{m}}{\gamma_{i,i}^{m}}$where γ_(i,j) ^(m) is a channel gain in the ith wire connection causedby the further wire connection indexed by j at tone m, and γ_(i,j) ^(m)is a channel gain along the ith channel at tone m. The step ofidentifying may comprise using the determined values of Γ_(i,j) ^(m).

The step of identifying may comprise, for each of the one or more tonesand for the further wire connection indexed by j, determining a valueof:

$\overset{\hat{}}{i} = {\arg{\max\limits_{i}\left\{ {\mu_{i,j}^{m} ⩓ \Gamma_{i,j}^{m}} \right\}}}$

wherein î denotes the identified wire connection that most stronglyreceives crosstalk interference from the further wire connection j atthat tone m.

The method may further comprise, for each of the one or more tones,determining a power allocation for transmitting a signal on the furtherwire connection at that tone, and transmitting a second signal onto thefurther wire connection at a tone using the determined power allocation.

Determining the power allocation may comprise determining, for each ofthe one or more tones indexed by m, and for the further wire connectionindexed by j, a value of:

$S_{\hat{i},j}^{m} = {\rho_{\hat{i},j}^{m}\left\lbrack {\frac{\left( {1 - \beta_{\begin{matrix}\max \\{\hat{i},j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{\hat{i},j}\end{matrix}}^{m}} \right)}{\left( {\Omega_{\overset{\hat{}}{i},j} + \eta_{\overset{\hat{}}{i},j}^{m}} \right)\;\ln\mspace{11mu} 2} - \frac{1}{\gamma_{\overset{\hat{}}{i},j}^{m}}} \right\rbrack}$where î denotes the identified wire connection that most stronglyreceives crosstalk interference from the further wire connection j atthat tone m; s_(î,j) ^(m) is a power allocation for transmitting asupporting signal for the î wire connection on the further wireconnection j at that tone m;

$\rho_{\overset{\hat{}}{i},j}^{m},\Omega_{\hat{i},j},\eta_{\hat{i},j}^{m},\beta_{\begin{matrix}\max \\{\hat{i},j}\end{matrix}}^{m},{{and}\mspace{14mu}\beta_{\begin{matrix}\min \\{\hat{i},j}\end{matrix}}^{m}}$are variables; and γ_(i,j) ^(m) is a channel gain in the wire connectionî caused by the further wire connection indexed by j at tone m.

The method may further comprise performing a water filling algorithm tojointly optimise a power distribution across the determined allocationof supporting signals.

In a further aspect, the present invention provides apparatus for use ina communication system, the communication system comprising atransmitter device and one or more receiver devices, each of the one ormore receiver devices being connected to the transmitter device via arespective wire connection, the transmitter device being operable totransmit signals onto the wire connections at one or more differenttones, the transmitter device further being operable to transmit signalsonto a further wire connection at the one or more different tones. Theapparatus comprises: measurement means configured to, for each of theone or more tones, measure, for each of the wire connections, anelectromagnetic coupling between the further wire connection and thatwire connection for that tone; and one or more processors configured to:for each of the one or more tones, using the electromagnetic couplingmeasurements, identify a wire connection that most strongly receivescrosstalk interference from the further wire connection at that tone;based on the identifications, for each of the one or more tones,allocate signals transmitted on the further wire connection at that toneas supporting signals for a particular wire connection; and operate thecommunication system to, for one or more of the tones, transmit a firstsignal onto the particular wire connection that has been allocated asupporting signal at that tone, and transmit a second signal onto thefurther wire connection at that tone, thereby to cause crosstalkinterference in the particular wire connection transmitting the firstsignal.

In a further aspect, the present invention provides a program orplurality of programs arranged such that when executed by a computersystem or one or more processors it/they cause the computer system orthe one or more processors to operate in accordance with the method ofany of the preceding aspects.

In a further aspect, the present invention provides a machine readablestorage medium storing a program or at least one of the plurality ofprograms according to the previous aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be better understood,embodiments thereof will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic illustration (not to scale) of an examplebroadband deployment;

FIG. 2 is a schematic illustration (not to scale) showing furtherspecific details of the broadband deployment;

FIG. 3 is a process flow chart showing certain steps of a datatransmission method;

FIG. 4 is a process flow chart showing certain steps of a frequencysplitting process of the data transmission method;

FIG. 5 is a process flow chart showing certain steps of a powersplitting process of the data transmission method; and

FIG. 6 is a process flow chart showing certain steps of a jointoptimisation process of the data transmission method.

SPECIFIC DESCRIPTION OF EMBODIMENTS

In the below description, reference will be made to modes ofcommunication. Herein, the term “mode” is used to indicate the nature ofthe manner in which signals are transmitted between transmitter andreceiver. In particular, as will be appreciated by persons skilled inthe art, there are three principal such modes of communication:differential mode, phantom mode, and common mode. In all three of thesemodes the signal is transmitted (excited) and received (observed) as the(changing) potential difference (voltage differential) between twovoltages (or equivalently between one “live” voltage and one “reference”voltage). In the differential mode the signal is transmitted/observed asthe difference in potential between two wires (typically between twowires of a twisted metallic pair). In the phantom mode at least one ofthe voltages is the average voltage of a pair of wires (note that suchaverage can vary without impacting on a signal carried in thedifferential mode across that same pair of wires—in this sense thephantom mode can be orthogonal to signals carried in the differentialmode if carefully chosen); the term pure phantom mode may be used tospecify that both voltages being compared with each other are averagevoltages, each average voltage being the average or common voltage of atleast one pair of wires. Second and higher order phantom modes can alsobe obtained by using the average voltage of two or more average voltagesas one of the voltages to be compared, etc. Finally, the common moderefers to the case where one of the voltages being compared is the“Earth” or ground reference voltage (or something substantially similarfor telecommunications purposes). Naturally, it is possible for variousmixed modes to also be used for carrying signals. For example, onereference voltage could be a common ground and the other could be theaverage between the voltages of two wires in a twisted metallic pair (togenerate a mixed mode of phantom and common modes). However, in general,reference to a differential mode in this specification is used to referto a pure differential mode, i.e. it does not include any phantom orcommon mode component so a mode comprising a comparison between thevoltage on a single wire and the average voltage between the voltages oftwo other wires may be referred to as an impure phantom mode rather thana mixed phantom and differential mode, etc.

Phantom channels can be constructed from different combinations of TMPs.For instance, a first and a second TMP can together generate a singleunique phantom channel which has a similar behaviour to that of eachdirectly coupled differential mode channel formed across each pair interms of channel directivity. However, phantom modes, as mentionedearlier, are due to the variation of the average voltages of the pairs.For more than two coupled pairs, the pairs may couple to each other inthe phantom mode in various orthogonal and non-orthogonal manners, e.g.two distinct (but non-orthogonal) phantom mode channels may be exploitedwhich share one common pair. Preferred embodiments of the inventionselect and construct only orthogonal phantom channels.

Reference is also made throughout the below description to direct andindirect coupling and direct and indirect channels. A direct channel isone in which the same physical medium and the same mode of transmissionis used for both the transmission of the signal and for the reception ofthe signal. Thus a normal differential mode transmission across a singleTMP from transmitter to receiver would constitute a direct (differentialmode) channel between the transmitter and the receiver. By contrast, achannel in which the transmitter transmitted a signal onto a second TMPin differential mode but was received by a receiver from a first TMP indifferential mode (the signal having “crosstalked” across from thesecond to the first pair) is an example of an indirect channel, as is acase in which a signal is transmitted by a transmitter in a phantom modeacross the averages of the voltages of the wires in each of a first andsecond TMP and received (having “crosstalked/mode” converted) by areceiver connected to just the first TMP in differential mode.

Moreover, where there are multiple TMPs emanating from a singletransmitter (e.g. an Access Node (AN) or Digital Subscriber Line AccessMultiplexor (DSLAM), etc.) in such a way that multiple direct andindirect channels are formed between the transmitter and multiplereceivers, the set of twisted metallic channel pairs and theirderivative channels (direct and indirect and of various different modes)can be considered as forming a “unified” dynamic shared or compositechannel over which a number of virtual channels may be overlaid (i.e.the virtual channels are overlaid over the underlying common sharedchannel). In this context, a virtual channel can be considered as anoverlay channel by which data can be directed to individual receiverseven though a single common underlying signal is transmitted onto theunderlying common channel; this can be achieved for example by means ofa suitable multiple access technique such as Frequency Division MultipleAccess (FDMA), Code Division Multiple Access (CDMA), Time DivisionMultiple Access (TDMA) or simply be using suitable encryptiontechniques, etc.

Referring now to the Figures, FIG. 1 is a schematic illustration (not toscale) of an example broadband deployment in which embodiments of a datatransmission method may be employed.

In this example, the deployment comprises a Distribution Point Unit(DPU) 10 which is connected to three user premises 31, 32, 33 (which inthis example are flats within a single building 30) via respectiveTwisted Metallic Pair (TMP) connections, namely a first TMP connection21, a second TMP connection 22, and a third TMP connection 23. The TMPconnections 21, 22, 23 connect between an Access Node (AN) 16 (whichmay, for example, be a DSLAM) within the DPU 10 and respective CustomerPremises Equipment (CPE) modems 51, 52 via respective networktermination points 41, 42 within the respective user premises 31, 32.

In this example, the deployment further comprises a plurality of furtherTMP connections 24-28, namely a fourth TMP connection 24, a fifth TMPconnection 25, a sixth TMP connection 26, a seventh TMP connection 27,and an eighth TMP connection 28. In this example, each further TMPconnection 24-28 is connected at one of its ends to the AN 16 such thata signal may be transmitted (and/or received) along that further TMPconnection 24-28 by the AN 16. Also, each further TMP connection 24-28may be, at its end distal with respect to the AN 16, either be connectedto a respective receiver (and/or transmitter), or be disconnected fromany electronic device. In this example, the TMP connections 21-28 arecontained within a common binder (not shown in the Figures) over atleast a portion of their lengths. For example, the TMP connections 21-28may be contained within the common binder at least between a point at orproximate to where the TMP connections 21-28 exit from/enter to the DPU10 and a point at which the first to third TMP connections 21, 22, 23diverge from the further TMP connections 24-28 to connect to therespective user premises 31, 32, 33. In this example, the binder is aflexible, electrically non-conductive casing or sheath (e.g. made ofplastics) that surrounds, and holds together the bundle of TMPconnections 21-28.

In this example, the DPU 10 additionally includes an Optical NetworkTermination (ONT) device 14 which provides a backhaul connection fromthe DPU 10 to a local exchange building via an optical fibre connectionsuch as a Passive Optic-fibre Network (PON) and a controller 12 whichco-ordinates communications between the AN 16 and the ONT 14, and whichmay perform some management functions such as communicating with aremote Persistent Management Agent (PMA).

As will be apparent to a person skilled in the art, the illustrateddeployment involving an optical fibre backhaul connection from adistribution point and a twisted metallic pair connection from thedistribution point to the “customer” premises is a sort of deploymentfor which the G.FAST standard is intended to be applicable. In such asituation, the TMP connections may be as short as a few hundred metresor less, for example possibly a few tens of metres only, and because ofthis it tends to be possible to use very high frequency signals (e.g. upto a few hundred Megahertz) to communicate over the short TMPs becausethe attenuation of high frequency signals is insufficient to preventthem from carrying useful information because of the shortness of thechannels. However, at such high frequencies crosstalk can become asignificant issue. This tends to be the case where the cross-talkingchannels travel alongside each other for part of their extent (as in thesituation illustrated in FIG. 1); however, cross-talk is also tends tobe an issue at high frequencies (e.g. over 80 MHz) even where thechannels only lie close to one another for a very small portion of theirtotal extent (e.g. just when exiting the DPU 10). G.fast proposes simplyusing vectoring techniques at all frequencies where there arecross-talking channels in order to mitigate against the cross-talkeffects.

In some embodiments, the DPU 10 (for example, the AN 16) exploitssignals (which may be considered to be supporting signals) transmittedonto the further TMP connections 24-28 and/or phantom channels whichwill “crosstalk” onto the conventional differential mode channelsassociated with each of the end user receivers (the termination pointand CPE modem combinations 41/51, 42/52, 43/53), and changes the signalsreceived (compared to a conventional case where the further TMPconnections 24-28 and phantom channels are not exploited in this way).The present embodiment includes a Phantom Channel-Multiple OptimisationProblem device (PC-MOP) which, as is explained in greater detail below,acts to select, for each one of the further TMP connections 24-28 andfor each tone, a particular one of the direct mode channel to that willreceive crosstalk interference from supporting signals transmitted onthat further TMP connection 24-28 at that tone. The selection may beperformed such as to try to achieve a particular set of two (or more)objectives.

FIG. 2 is a schematic illustration (not to scale) showing furtherdetails of the AN 16 and CPE modems 51, 52, 53 that allow for datatransmission according to below described embodiments.

The AN 16 comprises first, second, and third Data Source, Data Encoderand Serial to Parallel converter (DSDESP) modules 1611, 1612 and 1613.These are essentially conventional functions within a DSL modem and willnot be further described here except to point out that each one's outputis a set of data values d₁-d_(M) each of which can be mapped to both aset of one or more bits and to a point within a modulation signalconstellation associated with a respective tone on which the data valueis to be transmitted. For example, if a tone t₁ is determined to be ableto carry 3 bits of data, a corresponding data value will be set to oneof 2³=8 different values (e.g. to a decimal number between 0 and 7) eachof which corresponds to a different constellation point within anassociated signal constellation having 8 different constellation points.The data values for a single symbol can be thought of as forming avector of data values (one for each data-carrying tone) and togethercarry the user data to be transmitted to the end user associated with arespective end user modem 51, 52, 53 together with any overhead data(e.g. Forward Error Correction data etc.).

The data values leaving each DSDESP module 1611, 1612, 1613 are thenpassed (in an appropriate order) to respective Multiple bit levelQuadrature Amplitude Modulation (M-QAM) modulators 1621, 1622, 1623which convert each input data value to a respective complex number x₁ ¹to x_(M) ¹, x₁ ² to x_(M) ², and x₁ ³ to x_(M) ³ each of whichrepresents a complex point within a complex number constellationdiagram. For example, a data value d₁ ¹=7 (=111 in binary) might bemapped by the M-QAM modulator 1621 to the complex number 1-i for tone 1where tone 1 has been determined (by say modem 51) to be able to carry 3bits of data each.

Each of these complex numbers x₁ ¹ to x_(M) ¹, x₁ ² to x_(M) ², and x₁ ³to x_(M) ³ is then entered into a vectoring precoder module 1630 (whichin the present embodiment is a single common vectoring precoder module1630) which performs a largely conventional vectoring operation in orderto precode the transmissions to be sent using a combination ofpredetermined vectoring coefficients and information about the signalsto be transmitted onto the other channels within the relevant vectorgroup in a manner, which is well known to those skilled in the art, tocompensate for the expected effects of cross-talk from the otherchannels in the vector group.

In some embodiments, the vectoring precoder module 1630 is operable toadditionally precode the transmissions in such a way as to cause them tobe pre-compensated for the expected crosstalk effects produced not onlyby the neighbouring channels operating in a direct differential mode (asper standard vectoring), but also for the effects of crosstalk comingfrom any signals being transmitted onto one or more phantom channels (orother channels which are not direct differential mode channels). Inorder to do this, the vectoring precoder module 1630 may receiveinformation about channel estimations of the respective phantomchannel(s) (or other channels which are not direct differential modechannels) and also information about any weighting values used tocombine signals to be transmitted over the phantom channel(s) (or otherchannels which are not direct differential mode channels).

An ability of the vectoring precoder module 1630 to receive theweighting values and channel estimation values, which it may use toperform its precoding functions, is illustrated in FIG. 2 by the linebetween the PC-MOP & MICOP & MRC & Management entity module 1690 (whichperforms general management functions in addition to its specificfunctions described in greater detail below and, for brevity, mayhereinafter be referred to either as the “management entity” or the“PC-MOP module”) and the vectoring precoder module 1630. In thisembodiment, the PC-MOP module 1690 calculates appropriate values for thechannel estimations and the weighting values required by the vectoringprecoder module 1630 and the MICOP & MRC precoder module 1640. To dothis, the PC-MOP module 1690 may use data reported back to it from theend user modems 51, 52, 53. The processes and procedures for achievingthis are largely conventional and well known to persons skilled in theart and so they are not discussed in great detail herein except to notethat it may utilise a backward path from the user modems 51, 52, 53 tothe AN 16. This may be achieved in practice in that the user modems 51,52, 53 are transceivers capable of receiving and transmitting signalsover the TMPs 21, 22, 23 as is the AN 16. The receiver parts of the AN16 and the transmitter parts of the user modems 51, 52, 53 have simplybeen omitted from the drawings to avoid unnecessary complication of thefigures and because these parts are entirely conventional and notdirectly pertinent to the present invention. Moreover, each of the usermodems 51, 52, 53 may additionally contain a management entityresponsible for performing various processing and communicationfunctions. Any of a multitude of suitable techniques can be employed forobtaining data useful in generating channel estimations. For example,known training signals can be transmitted onto selected channels by theAN 16 during a special training procedure and the results of detectingthese by the user modems 51, 52, 53 can be sent back to the AN 16 in aconventional manner. Additionally, special synchronisation symbols canbe transmitted, interspersed with symbols carrying user data, atpredetermined “locations” within a “frame” comprising multiple symbols(e.g. at the beginning of each new frame) and the results of attemptingto detect these synchronisation symbols can also be sent back to the AN16 to generate channel estimation values. As is known to persons skilledin the art, different synchronisation signals/symbols can be sent overdifferent channels simultaneously and/or at different times etc. so thatdifferent channel estimations (including importantly indirect channelsand indirect channels can be targeted and evaluated, etc.

In this embodiment, the output from the vectoring precoder module 1630is a set of further modified complex numbers {circumflex over (x)}₁ ¹ to{circumflex over (x)}_(M) ¹, {circumflex over (x)}₁ ² to {circumflexover (x)}_(M) ², and {circumflex over (x)}₁ ³ to {circumflex over(x)}_(M) ³. These complex numbers are then passed to a Mixed-IntegerConvex Optimisation Problem and Maximal Ratio Combiner (MICOP and MRC)precoder module 1640 (hereinafter referred to as the MICOP and MRCprecoder module 1640). The MICOP and MRC precoder module 1640 may beconsidered to be a Maximal Ratio Transmission (MRT) module. In thepresent embodiment, the MICOP and MRC precoder module 1640 usesweighting values together with channel estimation values provided to itby the PC-MOP module 1690 to calculate, from the modified complexnumbers received from the vectoring pre-coder module 1640 (and theweighting values and channel estimation values from the PC-MOP module1690), further modified (or further pre-distorted) values for thecomplex numbers to be passed to the IFFTs 1651-1652. Thus, MICOP and MRCprecoder module 1640 modifies the received numbers {circumflex over(x)}₁ ¹ to {circumflex over (x)}_(M) ¹, {circumflex over (x)}₁ ² to{circumflex over (x)}_(M) ², and {circumflex over (x)}₁ ³ to {circumflexover (x)}_(M) ³ to generate corresponding further modified complexnumbers {umlaut over (x)}₁ ¹ to {umlaut over (x)}_(M) ¹, {umlaut over(x)} to {umlaut over (x)}_(M) ², and {umlaut over (x)}₁ ³ to {umlautover (x)}_(M) ³ which form (ultimately) the signals to be used indriving the respective TMPs 21, 22, 23 in direct differential mode.

Also, the MICOP and MRC precoder module 1640 may additionally generateone or more new sets of complex numbers, for example {umlaut over (x)}₁⁴ to {umlaut over (x)}_(M) ⁴, {umlaut over (x)}₁ ⁵ to {umlaut over(x)}_(M) ⁵, {umlaut over (x)}₁ ⁶ to {umlaut over (x)}_(M) ⁶, {umlautover (x)}₁ ⁷ to {umlaut over (x)}_(M) ⁷, and {umlaut over (x)}₁ ⁸ to{umlaut over (x)}_(M) ⁸, for forming (ultimately) the signals to be usedto drive a respective further TMP 24-28, or a respective (single ended)phantom mode channel, to be accessed via the MPAD module describedbelow. (In FIG. 2, only one of these new sets of complex numbers, namely{umlaut over (x)}₁ ⁴ to {umlaut over (x)}_(M) ⁴, is depicted for reasonsof clarity and ease of understanding of the Figures. However, it will beappreciated by the skilled person that, in practice, multiple of thesenew sets of complex numbers may be generated by the MICOP and MRCprecoder module 1640, and may then be sent to a respective IFFT module.)

Any appropriate way of generating the sets of complex numbers ({umlautover (x)}₁ ¹ to {umlaut over (x)}_(M) ¹, etc.) may be performed. Forexample, the method of transmitting data in differential and phantommodes that is described in WO 2016/139156 A1, which is incorporatedherein in its entirety, may be implemented. Once these values have beencalculated by the MICOP and MRC precoder 1640 they are passed to therespective IFFT modules 1651-1654, with super-script 1 values going toIFFT 1651, superscript 2 values going to IFFT 1652, and so on. The nexttwo steps of the processing are conventional and not relevant to thepresent invention. Thus, each set of generated values (e.g. {umlaut over(x)}₁ ¹ to {umlaut over (x)}_(M) ¹) is formed by the respective IFFTmodule into a quadrature time domain signal in the normal manner inOrthogonal Frequency Division Multiplexing (OFDM)/DMT systems.

The time domain signals are then processed by a suitable Analogue FrontEnd (AFE) module 1661 to 1664 again in any suitable such mannerincluding any normal conventional manner. After processing by the AFEmodule 1650, the resulting analogue signals are passed to a MultiplePhantom Access device (MPAD) module 1670. In overview, the MPAD module1670 provides switchable access to centre taps of any of the TMPs suchthat any of the possible phantom channels associated with the connectedchannels can be driven by the incoming signal arriving from AFE 1664 aswell as directly passing on the signals from AFEs 1661-1663 directly toTMPs 21-23 for driving in the normal direct differential mode.

During transmission over the TMP connections 21, 22, 23 the signals willbe modified in the normal way according to the channel response of thechannel and due to external noise impinging onto the connections. Inparticular, there will typically be crosstalking (including, forexample, far-end crosstalking) between the three direct channels (thedirect channels being one from the transmitter 16 to the modems 41-43via the TMPs 21-23), the further channels provided by the further TMPconnections 24-28, and the phantom channels. However, the effect of theprecoding is to largely precompensate for the effects of the crosstalk.Additionally, the targeted receivers may benefit from increased SNR ofthe received signal destined for them arriving via crosstalk from one ormore of the further TMP connections 24-28, and/or the phantom channel.

After passing over the TMP connections 21, 22, 23 the signals arereceived by the modems 41-43 at a respective Analogue Front End (AFE)module 5150, 5250, 5350 which performs the usual analogue front endprocessing. The thus processed signals are then each passed to arespective Fast Fourier Transform (FFT) module 5140, 5240, 5340 whichperforms the usual conversion of the received signal from the timedomain to the frequency domain. The signals leaving the FFT modules5140, 5240, 5340, y₁ ¹ to y_(M) ¹, y₁ ² to y_(M) ², and y₁ ³ to y_(M) ³,are then each passed, in the present embodiment, to a respectiveFrequency domain EQualiser (FEQ) module 5130, 5230, 5330. The operationof such frequency domain equaliser modules 5130, 5230, 5330 iswell-known in the art and will not therefore be further describedherein. It should be noted however, that any type of equalisation couldbe performed here, such as using a simple time-domain linear equalizer,a decision feedback equaliser, etc. For further information onequalisation in OFDM systems, the reader is referred to: “Zero-ForcingFrequency-Domain Equalization for Generalized DMT Transceivers withInsufficient Guard Interval,” by Tanja Karp, Steffen Trautmann, NorbertJ. Fliege, EURASIP Journal on Applied Signal Processing 2004:10,1446-1459.

Once the received signal has passed through the AFE, FFT and FEQmodules, the resulting signals {umlaut over (x)}₁ ¹ to {umlaut over(x)}_(M) ¹, {umlaut over (x)} to {umlaut over (x)}_(M) ², and {umlautover (x)}₁ ³ to {umlaut over (x)}_(M) ³ tend to be similar to thecomplex numbers x₁ ¹ to x_(M) ¹, x₁ ² to x_(M) ², and x₁ ³ to x_(M) ³originally output by the M-QAM modules 1621-1623, except that there maybe some degree of error resulting from imperfect equalisation of thechannel and the effect of external noise impinging onto the channelsduring transmission of the signals between the AN 16 and the modems 51,52, 53. This error will in general differ from one receiving modem tothe next. This can be expressed mathematically as {umlaut over (x)}_(m)¹=x_(m) ¹+e_(m) ¹ etc. Provided the error however is sufficiently smallthe signal should be recoverable in the normal way after processing bythe M-QAM demodulator modules 5120-5320 where a correspondingconstellation point is selected for each value {umlaut over (x)}_(m)^(i) depending on its value (e.g. by selecting the constellation pointclosest to the point represented by the value {umlaut over (x)}_(m) ^(i)unless trellis coding is being used, etc.). The resulting values {umlautover (d)}₁ ¹ to {umlaut over (d)}_(M) ¹, {umlaut over (d)}₁ ² to {umlautover (d)}_(M) ², and {umlaut over (d)}₁ ³ to {umlaut over (d)}_(M) ³should mostly correspond to the data values d₁ ¹ to d_(M) ¹, d₁ ² tod_(M) ², and d₁ ³ to d_(M) ³ originally entered to the correspondingM-QAM modules 1621, 1622, 1623 respectively within the AN 16. Thesevalues are then entered into a respective decoder (and received dataprocessing) module 5110, 5210 and 5230 which reassembles the detecteddata and performs any necessary forward error correction etc. and thenpresents the recovered user data to whichever service it is addressed toin the normal manner, thus completing the successful transmission ofthis data.

Following now from the above overview of FIG. 2, a more detailedexplanation is provided of the non-conventional elements within theembodiment illustrated in FIG. 2 and described briefly above.

In this embodiment, the PC-MOP module 1690 is a component configured todetermine, for each of the supporting further TMPs 24-28, and for eachtone on which data is transmitted, a receiving TMPs 21, 22, 23 toreceive crosstalk interference from that supporting further TMP 24-28 toincrease the SNR of a signal along that TMP 21, 22, 23 at that tone. TheMPAD 1670 may also accordingly operate the further TMPs 24-28 and/orphantom channels to improve the composed active channels and hence theSNR of signals along the TMPs 21, 22, 23. The PC-MOP module 1690 isfurther configured to determine bit and power allocations with which totransmit data signal to provide the increased SNRs of signals along theTMP 21, 22, 23. In particular, the PC-MOP module 1690 may be configuredto determine bit and power allocation such that the maximum capacitiesof the channels (i.e. the maximum transmission bit rates) aresubstantially achieved.

Apparatus, including the PC-MOP module 1690 and MPAD 1670, forimplementing the above arrangement, and performing the method steps tobe described later below, may be provided by configuring or adapting anysuitable apparatus, for example one or more computers or otherprocessing apparatus or processors, and/or providing additional modules.The apparatus may comprise a computer, a network of computers, or one ormore processors, for implementing instructions and using data, includinginstructions and data in the form of a computer program or plurality ofcomputer programs stored in or on a machine readable storage medium suchas computer memory, a computer disk, ROM, PROM etc., or any combinationof these or other storage media.

What follows is a mathematical explanation of the functioning of certainelements of the above described system. This mathematical explanation isuseful in the understanding of the embodiments of the methods ofincreasing the SNR of signals transmitted along the TMPs 21, 22, 23,which embodiments are described in more detail later below withreference to FIGS. 3 to 5.

Considering a system comprising a bundle of k TMPs and/or phantomchannels, the full transmission characteristics for a single frequencyof the differential mode channel may be represented as:

$H = \begin{pmatrix}h_{1,1} & h_{1,2} & \Lambda & h_{1,k} \\h_{2,1} & h_{2,2} & \Lambda & h_{2,k} \\M & M & O & M \\h_{k,1} & h_{k,2} & \Lambda & h_{k,k}\end{pmatrix}$

where h_(i,j) indicates a channel transfer function, a value of which isdependent on crosstalk transmissions from the jth TMP/phantom channelonto the ith TMP/phantom channel. In other words, h_(i,j) is a measureof the electromagnetic coupling of the jth channel to the ith channel.For example, h_(i,j) may be indicative of the extent of coupling betweenthe jth channel and the ith channel. Values of h_(i,j) may be dependenton an attenuation on the amplitude of signals on the ith channel causedby the jth channel. Values of h_(i,j) may be dependent on a delay and/orphase shift on the phase of signals on the ith channel caused by the jthchannel.

In this embodiment, the channel transfer matrix H is known i.e.measured. For example, the channel transfer matrix H may be determinedusing known channel estimation techniques.

Values of h_(i,j) may be measured, for example, using test signals, bythe MPAD 1670. Measurement of the h_(i,j) values may comprise, or beregarded as equivalent to, measuring one or more of the followingparameters selected from the group of parameters consisting of: achannel response of the ith channel under conditions in which the jthchannel (e.g. only the jth channel) is transmitting crosstalkinterference onto the ith channel, an impulse response of the ithchannel under conditions in which the jth channel (e.g. only the jthchannel) is transmitting crosstalk interference onto the ith channel,and a frequency response of the ith channel under conditions in whichthe jth channel (e.g. only the jth channel) is transmitting crosstalkinterference onto the ith channel.

The bundle of k TMPs and/or phantom channels may be contained in acommon binder.

For convenience, herein a sub-matrix of H is considered, i.e.

$H_{sub} = \begin{pmatrix}h_{1,1} & h_{1,2} & h_{1,3} & \ldots & h_{1,8} \\h_{2,1} & h_{2,2} & h_{2,3} & \ldots & h_{2,8} \\h_{3,1} & h_{3,2} & h_{3,3} & \ldots & h_{3,8}\end{pmatrix}$

which represents cross-talk from each of the TMPs 21-28 to each of thefirst, second, and third TMPs 21, 22, 23. In other embodiments, adifferent sub-matrix or the full matrix H is considered.

As mentioned above, in operation, the PC-MOP module 1690 performs acrosstalk splitting and steering process to maximise aggregate bundlecapacity. To achieve this, the MICOP and MRC precoder module 1640 maygenerate the further modified complex numbers {umlaut over (x)}₁ ¹ to{umlaut over (x)}_(M) ¹, etc. to drive the TMPs 21-28 based on thefollowing factor:

$P_{MRC} = \begin{pmatrix}\frac{h_{1,1}^{*}}{h_{1,1}} & 0 & 0 & \ldots & \frac{\rho_{1,8}h_{1,8}^{*}}{h_{1,8}} \\0 & \frac{h_{2,2}^{*}}{h_{2,2}} & 0 & \ldots & \frac{\rho_{2,8}h_{2,8}^{*}}{h_{2,8}} \\0 & 0 & \frac{h_{3,3}^{*}}{h_{3,3}} & \ldots & \frac{\rho_{3,8}h_{3,8}^{*}}{h_{3,8}}\end{pmatrix}$

where ρ_(i,j) is a pair sharing and steering factor representative of aproportion of the power along the jth TMP that is allocated tosupporting transmission along the ith TMP. In this embodiment, theρ_(i,j) satisfy the following conditions: 0≤ρ_(1,j)+ρ_(2,j)+ρ_(3,j)≤1,∀j∈{4, . . . , 8} and ρ_(i,j)∈[0,1]. In some embodiments, ρ_(i,j) arebinary allocation factors, i.e. ρ_(i,j)∈{0,1}.

In this example, there is little to no interaction between the first,second, and third TMPs 21, 22, 23, for example as a result of the abovedescribed vectoring process. However, crosstalk interference from thefurther TMPs 24-28 onto the first, second, and third TMPs 21, 22, 23 maybe non-zero.

The maximum capacity of a channel (which may also be referred to asShannon's capacity limit) is expressed as:

$\begin{matrix}{C = {B\mspace{11mu}{\log_{2}\left( {1 + \frac{S}{N}} \right)}}} & (1)\end{matrix}$

where: C is the maximum capacity of the channel (in bits/second) for thegiven channel; B is the bandwidth of the channel (in Hertz); S is thesignal power (in Watts); and N is the noise power (in Watts). The ratioS/N is called the Signal to Noise Ratio (SNR).

Thus, the crosstalk splitting and steering process to maximise aggregatebundle capacity that is performed by the PC-MOP module 1690 is that of,for each of the further TMPs 24-28 (and phantom channels in someembodiments), maximising the following objective function:

$\begin{matrix}{C_{j} = {\sum\limits_{{m = 1},{i = {{1j} = 1}}}^{{m = M},{i = 3},{j = 8}}{\rho_{i,j}^{m}{\log_{2}\left( {1 + \frac{s_{i,j}^{m}\gamma_{i,j}^{m}}{\rho_{i,j}^{m}}} \right)}}}} & (2)\end{matrix}$

where: i is an index for the target receivers and the associateddirect-channel TMPs, i.e. i indexes channels receiving crosstalkinterference from channel j. In this embodiment, the first, second, andthird channels are to receive crosstalk interference from the furtherTMPs 24-28. Thus i=1, . . . , 3;

-   -   j is an index for TMPs acting as transmitters of crosstalk        interference. Thus, j=1, . . . 8;    -   m is a frequency index for the M different tones, m=1, . . . M;    -   ρ_(i,j) ^(m) is a pair sharing factor indicating, for tone m,        the proportion of transmit power along the jth TMP that is        allocated to crosstalk interfere with the ith TMP;    -   s_(i,j) ^(m) is a power level of crosstalk interference        transmitted from the jth channel onto the ith channel, at the        mth tone; and    -   γ_(i,j) ^(m) is a channel gain in the ith channel caused by        crosstalk interference from the jth channel. In this embodiment,        γ_(i,j) ^(m) is a ratio of power coupling coefficient to the        noise level at the mth tone, i.e.

${\gamma_{i,j}^{m} = \left( \frac{\left| h_{i,j} \right|^{2}}{n_{i,i}} \right)_{m}},$where n_(i,i) is a noise level of the ith channel for given tone m. Thechannel gain γ_(i,j) ^(m) can also include other factors, for exampleone or more factors selected from the group of factors consisting of:coding gain, margin, gap value, and deterministic noise signals.

In this embodiment, the above objective function (equation (2)) isoptimised with respect to the following constraints:

$\begin{matrix}{{\Delta_{f}{\sum\limits_{m,i}\; s_{i,j}^{m}}} \leq P_{T}} & (3) \\{{{\sum\limits_{i}\rho_{i,j}^{m}} \leq 1},{\forall m}} & (4) \\{{s_{i,j}^{m} \leq p_{m}},{\forall m}} & (5) \\{b_{\min} \leq {\rho_{i,j}^{m}{\log_{2}\left( {1 + \frac{s_{i,j}^{m}\gamma_{i,j}^{m}}{\rho_{i,j}^{m}}} \right)}} \leq b_{\max}} & (6)\end{matrix}$

where: P_(T) is the maximum transmitting power (also known as theAggregate Transmit Power (ATP)) permitted for the bundle of k TMPsand/or phantom channels;

-   -   p_(m) is a transmission power mask for tone m. In this        embodiment, the power mask is an upper threshold for power for        signal transmission at tone m above which power value        transmission is not permitted. The transmission power mask(s)        may be set, for example, by an official regulatory body;    -   b_(min) is a lower bound for the channel capacity. In this        embodiment, b_(min) is set to zero (0). However, in other        embodiments, b_(min) may have a different appropriate value;    -   b_(max) is an upper bound for the channel capacity. In this        embodiment, b_(max) is a bit limit for the TMPs and/or phantom        channels.

Values for b_(min) and/or b_(max) may result from hardware limitations.

The above objective function (equation (2)) is a concave objectivefunction that is to be maximised subject to the constraints in equations(3)-(6). Since the objective function (equation (2)) is concave, itsoptimisation is tractable. The optimisation of the objective function(equation (2)) proceeds with the Lagrangian as follows:

$\begin{matrix}{\mathcal{L} = {{\sum\limits_{m,i,j}{\rho_{i,j}^{m}{\log_{2}\left( {1 + \frac{s_{i,j}^{m}\gamma_{i,j}^{m}}{\rho_{i,j}^{m}}} \right)}}} - {\Omega_{i,j}\left( {{\sum\limits_{m,i}s_{i,j}^{m}} - P_{T}} \right)} - {\sum\limits_{m}{\mu_{i,j}^{m}\left( {{\sum\limits_{i}\rho_{i,j}^{m}} - 1} \right)}} - {\sum\limits_{m}{\eta_{i,j}^{m}\left( {{\sum\limits_{i}s_{i,j}^{m}} - p_{m}} \right)}} - {\sum\limits_{m}{\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}\left\lbrack {{\sum\limits_{i}{\rho_{i,j}^{m}{\log_{2}\left( {1 + \frac{s_{i,j}^{m}\gamma_{i,j}^{m}}{\rho_{i,j}^{m}}} \right)}}} - b_{\max}} \right\rbrack}} - {\sum\limits_{m}{\beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}\left\lbrack {{\sum\limits_{i}{\rho_{i,j}^{m}{\log_{2}\left( {1 + \frac{s_{i,j}^{m}\gamma_{i,j}^{m}}{\rho_{i,j}^{m}}} \right)}}} - b_{\min}} \right\rbrack}}}} & (7)\end{matrix}$

where

${\Omega_{i,j},\eta_{i,j}^{m},\mu_{i,j}^{m},\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m},{{and}\mspace{14mu}\beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}}}\mspace{11mu}$are Lagrangian multipliers.

To solve (7) and to show its optimality, in this embodiment the KarushKuhn Tucker (KKT) conditions are satisfied. These conditions are asfollows:

1. Feasibility of the primal constraints as well as the multipliers,i.e.

$\Omega_{i,j},\eta_{i,j}^{m},\mu_{i,j}^{m},\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m},{{{and}\mspace{14mu}\beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \geq 0.}$

2. The gradient of the Lagrangian (equation (7)) with respect to s, andthe gradient of the Lagrangian (equation (7)) with respect to ρ bothbecome zero.

Differentiating the Lagrangian (equation (7)) with respect to s, i.e. d

/ds=0, and rearranging for s gives the optimal power formula:

$\begin{matrix}{s_{i,j}^{m} = {{p_{i,j}^{m}\left\lbrack {\frac{\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)}{\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)\ln\mspace{14mu} 2} - \frac{1}{\gamma_{i,j}^{m}}} \right\rbrack}\left( {{Watt}/{Hz}} \right)}} & (8)\end{matrix}$

Differentiating the Lagrangian (equation (7)) with respect to ρ, i.e. d

/dρ=0, and rearranging for μ gives:

$\begin{matrix}{\mu_{i,j}^{m} = {\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} -}\quad \right.\left. \quad\beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m} \right)\left\{ {{\log_{2}\left\lbrack \frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)}{\ln\mspace{14mu} 2\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)} \right\rbrack} - {{\quad\quad}\left\lbrack {\frac{1}{\ln\mspace{14mu} 2} - \frac{\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} - \beta_{\begin{matrix}\min \\{i,j}\end{matrix}}^{m}} \right)}} \right\rbrack}} \right\}}} & (9)\end{matrix}$

What will now be described with reference to FIG. 3 is an embodiment ofa data transmission method in which indirect channels (e.g. the furtherTMPs 24-28 in this embodiment, but also the phantom channels in otherembodiments) are operated to improve the SNR of signals along the first,second, and third TMPs 21, 22, 23. Advantageously, the data transmissiontends to achieve substantially maximum channel capacity. In thisembodiment, this method is performed by the MPAD 1670 and/or the PC-MOP1690. However, in other embodiments, the method is performed by adifferent entity instead of or in addition to the MPAD 1670 and/or thePC-MOP 1690.

FIG. 3 is a process flow chart showing certain steps of an embodiment ofthe data transmission method.

At step s2, the MPAD 1670 and/or the PC-MOP 1690 performs a so-calledfrequency splitting process. The frequency splitting process isdescribed in more detail later below with reference to FIG. 4. In thisembodiment, the frequency splitting process is performed to select, foreach of the further TMPs 24-28, and for each tone m, a respectivereceiver TMP 21-23 that is a preferred (e.g. optimal) receiver ofcrosstalk interference from that further TMPs 24-28 at that tone m.

At step s4, using the results of the frequency splitting process, theMPAD 1670 performs a so-called power splitting process. The powersplitting process is described in more detail later below with referenceto FIG. 5. In this embodiment, the power splitting process is performedto determine aggregate power distributions for the direct channel TMPs21-23.

At step s6, using the results of the frequency splitting process (steps2) and the power splitting process (step s4), the MPAD 1670 performs ajoint optimisation process to maximise overall channel capacity. Thejoint optimisation process performed at step s6 is a bit and powerallocation process. The joint optimisation process is described in moredetail later below with reference to FIG. 6. In this embodiment, theprocess performed at step s6 includes transmitting signals along theTMPs 21-28.

Thus, an embodiment of a data transmission method is provided.

Returning now to the description of step s2, FIG. 4 is a process flowchart showing certain steps of the frequency splitting process.

In this embodiment, b_(min)=0. Thus, also

$\beta_{\begin{matrix}\min \\{j,j}\end{matrix}}^{m} = 0.$However, in some embodiments, b_(min) may have a different, non-zerovalue. Also, in some embodiments,

$\beta_{\begin{matrix}\min \\{j,j}\end{matrix}}^{m}$may have a different, non-zero value.

At step s10, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M,

$\beta_{\begin{matrix}\max \\{j,j}\end{matrix}}^{m}\mspace{14mu}{and}\mspace{14mu}\eta_{i,j}^{m}$are initialised to zero, and ρ_(i,j) ^(m) is initialised to one. That isto say:

$\begin{matrix}{\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} = {\eta_{i,j}^{m} = 0}} & (10) \\{\rho_{i,j}^{m} = 1.} & (11)\end{matrix}$

At step s12, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8, avalue of Ω_(i,j) is computed. In this embodiment, Ω_(i,j) is computed byusing equations (8) and (3), with the parameters initialised as in steps10. In particular, equation (8) is plugged into equation (3) to give:

$\begin{matrix}{\Omega_{i,j} = \frac{M}{\ln\mspace{14mu} 2\left( {\frac{P_{T}}{\Delta_{f}} + {\sum\limits_{m}\frac{1}{\gamma_{i,j}^{m}}}} \right)}} & (12)\end{matrix}$

where: M is the number of different tones; and

-   -   Δ_(f) is a frequency spacing between adjacent tones.

At step s14, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M, a value for η_(i,j) ^(m) is computed.In this embodiment, η_(i,j) ^(m) is computed by using equation (8) andthe value for Ω_(i,j) computed at step s12. In particular, equation (8)is set equal to p_(m) (i.e. the transmission power mask for tone m), andthen rearranged for η_(i,j) ^(m) to give:

$\begin{matrix}{\eta_{i,j}^{m} = {\frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}} \right)}{\left( {\ln\mspace{14mu} 2} \right)\left( {1 + {p_{m}\gamma_{i,j}^{m}}} \right)} - \Omega_{i,j}}} & (13)\end{matrix}$

At step s16, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M, a value for

$\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}$is computed. In this embodiment,

$\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}$is computed using equation (8) and the equation log₂(1+s_(i,j)^(m)γ_(i,j) ^(m))=b_(max). In particular, equation (8) is plugged intolog₂(1+s_(i,j) ^(m)γ_(i,j) ^(m))=b_(max) and rearranged for

$\beta_{\max\limits_{i,j}}^{m}$to give:

$\begin{matrix}{\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} = {1 - \frac{2^{b_{\max}}\left( {\ln\; 2} \right)\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}}}} & (14)\end{matrix}$

At step s18, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M, a value for μ_(i,j) ^(m) is computedusing equation (9) and the computed and the values for Ω_(i,j), η_(i,j)^(m) and

$\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m}$computed at steps s12, s14, and s16 respectively.

At step s20, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M, a value of Γ_(i,j) ^(m) is computed.In this embodiment, Γ_(i,j) ^(m) is given by:

$\begin{matrix}{\Gamma_{i,j}^{m} = \frac{\gamma_{i,j}^{m}}{\gamma_{i,i}^{m}}} & (15)\end{matrix}$

where: γ_(i,j) ^(m) is a channel gain in the ith channel caused by thejth channel; and

-   -   γ_(i,j) ^(m) is a channel gain along the ith channel.

In this embodiment, the gains γ_(i,j) ^(m) and γ_(i,j) ^(m) aredetermined or measured by the MPAD 1670 (for example, from measuredvalues of h_(ij)) or by state of the art channel estimation in DSLdevices, e.g. vectoring control entity (VCE) channel feedback in G.fast.

At step s24, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, and for each tone m=1, . . . , M, a value of the function î isdetermined, where:

$\begin{matrix}{\overset{\hat{}}{i} = {\arg{\max\limits_{i}\left\{ {\mu_{i,j}^{m} ⩓ \Gamma_{i,j}^{m}} \right\}}}} & (16)\end{matrix}$

where the wedge symbol (Λ) denotes the logical conjunction (AND)operator.

In this embodiment, î is the index identifier of the direct channel TMPconnection 21-23, that most strongly receives crosstalk interferencefrom the jth further TMP connection 24-28, at the mth tone. In otherwords, î is an identifier of the channel acting as a receiver ofcrosstalk which is most strongly affected by, or crosstalk coupled to,signals transmitted on the jth channel at the mth tone.

At step s26, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, and for each tone m=1, . . . , M, a values of the allocation factor ρfor the direct-channel TMP identified by î is set equal to one, whilethe allocation factors of the other direct-channel TMPs 21-23 are setequal to zero, i.e.:ρ_(î,j) ^(m)=1  (17)ρ_(i,j) ^(m)=0, ∀i≠î  (18)

Thus, the frequency splitting process is provided. In this embodiment,for each of the further TMPs 24-28 and for each tone m, a direct-channelTMP 21-23 that is the best receiver of crosstalk interference from thatfurther TMP 24-28 at that tone m is determined and allocated to thatfurther TMP 24-28 and that tone m.

Returning now to the description of step s4, in this embodiment, thepower splitting process is performed to optimise power distribution inaccordance with the allocation determined at step s2 and described inmore detail above with reference to FIG. 3. In this embodiment, thepower splitting process is performed to maximise the following objectivefunction:

$\begin{matrix}{C_{j} = {\Delta_{f}{\sum\limits_{{m = 1},\hat{i}}^{m = M}{\rho_{\overset{\hat{}}{i},j}^{m}{\log_{2}\left( {1 + \frac{s_{\overset{\hat{}}{i},j}^{m}\gamma_{\overset{\hat{}}{i},j}^{m}}{\rho_{\hat{i},j}^{m}}} \right)}}}}} & (19)\end{matrix}$

where: j is an index for the indirect-channel TMPs 24-28, j=4, . . . 8;

-   -   m is a frequency index for the M different tones, m=1, . . . M;    -   î is the direct TMP selected for the jth indirect TMP and mth        tone, at step s2.

In this embodiment, the above objective function (equation (19)) isoptimised with respect to the following constraints:

$\begin{matrix}{{\Delta_{f}{\sum\limits_{m,\hat{i}}s_{\hat{i},j}^{m}}} \leq P_{T}} & (20) \\{{{\sum\limits_{\hat{i}}\rho_{\hat{i},j}^{m}} \leq 1},\mspace{14mu}{\forall m}} & (21) \\{{s_{\overset{\hat{}}{i},j}^{m} \leq p_{m}},\mspace{14mu}{\forall m}} & (22) \\{b_{\min} \leq {\rho_{\overset{\hat{}}{i},j}^{m}{\log_{2}\left( {1 + \frac{s_{\overset{\hat{}}{i},j}^{m}\gamma_{\hat{i},j}^{m}}{\rho_{\overset{\hat{}}{i},j}^{m}}} \right)}} \leq b_{\max}} & (23)\end{matrix}$

Optionally, in some embodiments, the right-hand side of Equation 21 canbe set equal to the number of the active receivers when vectoring isemployed, as opposed to 1.

FIG. 5 is a process flow chart showing certain steps of the powersplitting process.

In this embodiment, b_(min)=0. Thus, also

$\beta_{\begin{matrix}\min \\{j,j}\end{matrix}}^{m} = 0.$However, in some embodiments, b_(min) may have a different, non-zerovalue. Also, in some embodiments,

$\beta_{\begin{matrix}\min \\{j,j}\end{matrix}}^{m}$may have a different, non-zero value.

At step s30, for each of the direct TMPs 21-23 indexed by i=1, . . . ,3, and for each of the indirect TMPs 24-28 indexed by j=4, . . . , 8,and for each tone m=1, . . . , M,

$\beta_{\begin{matrix}\max \\{j,j}\end{matrix}}^{m}\mspace{14mu}{and}\mspace{14mu}\eta_{ij}^{m}$are initialised to zero. That is to say:

$\beta_{\begin{matrix}\max \\{i,j}\end{matrix}}^{m} = {\eta_{i,j}^{m} = 0}$

Also, in this embodiment, the allocation factors ρ are set as determinedat step s26.

At step s32, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, a value of Ω_(i,j) is computed, where î is the direct TMP selectedfor that jth indirect TMP at step s2. In this embodiment, Ω_(i,j) isdetermined as described earlier above at step s12, with i replaced by î,i.e.:

$\begin{matrix}{\Omega_{\overset{\hat{}}{i},j} = \frac{M}{\ln\; 2\left( {\frac{P_{T}}{\Delta_{f}} + {\sum\limits_{m}\frac{1}{\gamma_{\overset{\hat{}}{i},j}^{m}}}} \right)}} & (24)\end{matrix}$

At step s34, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, and for each tone m=1, . . . , M, a value for η_(î,j) ^(m) iscomputed, where î is the direct TMP selected for that jth indirect TMPand that tone m at step s2. In this embodiment, η_(î,j) ^(m) is computedby using equation (8) and the value for Ω_(î,j) computed at step s32. Inparticular, equation (8) is set equal to p_(m) (i.e. the transmissionpower mask for tone m), and then rearranged for η_(î,j) ^(m) to give:

$\begin{matrix}{\eta_{\overset{\hat{}}{i},j}^{m} = {\frac{\gamma_{\overset{\hat{}}{i},j}^{m}\left( {1 - \beta_{\begin{matrix}\max \\{\hat{i},j}\end{matrix}}^{m}} \right)}{\left( {\ln\; 2} \right)\left( {1 + {p_{m}\gamma_{\overset{\hat{}}{i},j}^{m}}} \right)} - \Omega_{\hat{i},j}}} & (25)\end{matrix}$

At step s36, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, and for each tone m=1, . . . , M, a value for

$\beta_{\begin{matrix}\max \\{\hat{i},j}\end{matrix}}^{m}$is computed, where î is the direct TMP selected for that jth indirectTMP and that tone m at step s2. In this embodiment,

$\beta_{\begin{matrix}\max \\{\hat{i},j}\end{matrix}}^{m}$is computed using equation (8) and the equation log₂(1+s_(i,j)^(m)γ_(i,j) ^(m))=b_(max). In particular, equation (8) is plugged intolog₂(1+s_(i,j) ^(m)γ_(i,j) ^(m))=b_(max) and rearranged for

$\beta_{\underset{\hat{i},j}{{ma}\; x}}^{m}$to give:

$\begin{matrix}{\beta_{\underset{\hat{i},j}{{ma}\; x}}^{m} = {1 - \frac{2^{b_{{ma}\; x}}\left( {\ln\; 2} \right)\left( {\Omega_{\hat{i},j} + \eta_{\hat{i},j}^{m}} \right)}{\gamma_{\hat{i},j}^{m}}}} & (26)\end{matrix}$

At step s38, for each of the indirect TMPs 24-28 indexed by j=4, . . . ,8, and for each tone m=1, . . . , M, a value for s_(î,j) ^(m) iscomputed, where î is the direct TMP selected for that jth indirect TMPand that tone m at step s2. The values for s_(î,j) ^(m) are computed byusing equation (8) and the values for Ω_(î,j), η_(î,j) ^(m) and

$\beta_{\underset{\hat{i},j}{{ma}\; x}}^{m}$computed at steps s32, s34, s36 respectively. In other words, values ofs_(î,j) ^(m) are determined, where:

$S_{\hat{i},j}^{m} = {\rho_{\hat{i},j}^{m}\left\lbrack {\frac{\left( {{1 -},{\beta_{\underset{\hat{i},j}{{ma}\; x}}^{m} - \beta_{\underset{\hat{i},j}{m\; i\; n}}^{m}}} \right)}{\left( {\Omega_{\hat{i},j} + \eta_{\hat{i},j}^{m}} \right)\ln\; 2} - \frac{1}{\gamma_{\hat{i},j}^{m}}} \right\rbrack}$

Optionally, at step s40, for each of the indirect TMPs 24-28 indexed byj=4, . . . , 8, and for each of the direct TMPs 21-23 selected asreceivers of crosstalk interference from that indirect TMP j (which areindexed by î), a value of the aggregate power distribution Pg_(î,j)between that indirect TMP j and that direct TMP î is determined, where

$\begin{matrix}{{{Pg}_{\hat{i},j} = {\Delta_{f}{\sum\limits_{m}s_{\hat{i},j}^{m}}}},{\forall\hat{i}}} & (27)\end{matrix}$

Thus, the power splitting process is provided. In this embodiment, foreach of the further TMPs 24-28 and for each tone m, a power allocationfor the direct-channel TMP 21-23 that is the best receiver of crosstalkinterference from that further TMP 24-28 at that tone m is determined,and is allocated to that further TMP 24-28 and that tone m.

At step s42, it is determined whether or not the determined powerallocations are less than or equal to the maximum permitted transmittingpower for the bundle of TMPs 21-28. In particular, it is determinedwhether or not the following equation is satisfied:

$\begin{matrix}{{\Delta_{f}{\sum\limits_{m = 1}^{M}s_{\hat{i},j}^{m}}} \leq P_{T}} & \left( {27a} \right)\end{matrix}$

In embodiments in which values for Pg_(î,j) were calculated at step s40,the calculated values for Pg_(î,j) may be used to check whether or notthe determined power allocations are less than the maximum permittedtransmitting power instead of or in addition to Equation 27a.

In this embodiment, if, at step s42, Equation 27a is satisfied (i.e. itis determined that the determined power allocations are less than orequal to the maximum permitted transmitting power for the bundle of TMPs21-28), the process of FIG. 5 proceeds to step s46.

However, if, at step s42, Equation 27a is not satisfied (i.e. it isdetermined that the determined power allocations are less than themaximum permitted transmitting power for the bundle of TMPs 21-28), theprocess of FIG. 5 proceeds to step s44.

At step s44, the value of Ω_(î,j) is increased (e.g. by a predeterminedamount) and the process of FIG. 5 returns to step s34.

At step s46, the AN 16 transmits signals in accordance with thedetermined allocated power values s_(î,j) ^(m). For example, for eachtone m, the AN 16 may transmit a signal having transmission powers_(î,j) ^(m) at tone m along the jth TMP.

Thus, the power splitting process is provided.

In the process of step s4 above, the bit loading limits at s4 may bechosen for convenience. In some embodiments, optionally the bit loadinglimits in s4 can be further optimised (e.g. using a heuristic process)to improve further the efficiency of the power contribution. Final bitloading may be decided in step s6, which will now be described.

Returning now to the description of step s6, in this embodiment, toperform the joint optimisation process to allocate bits and power to thechannels and tones, any appropriate so-called “water filling” algorithmmay be used. A known water filling algorithm may be adapted to accountfor the results of the above described frequency splitting process, i.e.the steered cross talk expressed by the allocation factors ρ. Also, thewater filling algorithm may be adapted to account for the results of theabove described power splitting process, i.e. the power distributionsexpressed by the determined values for s_(î,j) ^(m) and/or Pg_(î,j).

In this embodiment, the process of step s6 is performed to maximise thefollowing objective function:

$\begin{matrix}{C_{\hat{i}} = {\sum\limits_{m = 1}^{m = M}{\rho_{\hat{i},\hat{i}}^{m}{\log_{2}\left( {1 + \frac{s_{\hat{i},\hat{i}}^{m}\gamma_{\hat{i},\hat{i}}^{m}}{\rho_{\hat{i},\hat{i}}^{m}} + {\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}}} \right)}}}} & (28)\end{matrix}$

where:

$\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}$is the signal power that the î direct TMP receives from all of the otherTMPs (i.e. the TMPs that are not î).

In this embodiment, the above objective function (equation (28)) isoptimised with respect to the following constraints:

$\begin{matrix}{{\Delta_{f}{\sum\limits_{m}s_{\hat{i},\hat{i}}^{m}}} \leq P_{T}} & (29) \\{{{\sum\limits_{\hat{i}}\rho_{\hat{i},\hat{i}}^{m}} \leq 1},{\forall m}} & (30) \\{{S_{\hat{i},\hat{i}}^{m} \leq p_{m}},{\forall m}} & (31) \\{b_{m\; i\; n} \leq {\rho_{\hat{i},\hat{i}}^{m}{\log_{2}\left( {1 + \frac{S_{\hat{i},\hat{i}}^{m}\gamma_{\hat{i},\hat{i}}^{m}}{\rho_{\hat{i},\hat{i}}^{m}} + {\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}}} \right)}} \leq b_{{ma}\; x}} & (32)\end{matrix}$

Optionally, in some embodiments, the right-hand side of Equation 30 canbe set equal to the number of the active receivers when vectoring isemployed, as opposed to 1.

The optimisation problem defined by equations (29) to (32) gives thefollowing power formula:

$\begin{matrix}{\mspace{20mu}{{s_{\hat{i},\hat{i}}^{m} = {{\rho_{\hat{i},\hat{i}}^{m}\left\lbrack {\frac{\left( {1 - \beta_{\underset{\hat{i},\hat{i}}{{ma}\; x}}^{m} - \beta_{\underset{\hat{i},\hat{i}}{m\; i\; n}}^{m}} \right)}{\left( {\Omega_{\hat{i},\hat{i}} + \eta_{\hat{i},\hat{i}}^{m}} \right)\ln\; 2} - \frac{1 + {\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}}}{\gamma_{\hat{i},\hat{i}}^{m}}} \right\rbrack}\left( {{Watt}\text{/}{Hz}} \right)}}\mspace{20mu}{{and}\mspace{14mu}{also}\text{:}}}} & (33) \\{\mu_{\hat{i},\hat{i}}^{m} = {\left( {1 - \beta_{\underset{\hat{i},\hat{i}}{{ma}\; x}}^{m} - \beta_{\underset{\hat{i},\hat{i}}{m\; i\; n}}^{m}} \right)\left\{ {{\log_{2}\left\lbrack \frac{\gamma_{\hat{i},\hat{i}}^{m}\left( {1 - \beta_{\underset{\hat{i},\hat{i}}{{ma}\; x}}^{m} - \beta_{\underset{\hat{i},\hat{i}}{m\; i\; n}}^{m}} \right)}{\ln\; 2\left( {\Omega_{\hat{i},\hat{i}} + \eta_{\hat{i},\hat{i}}^{m}} \right)} \right\rbrack} - \left\lbrack {\frac{1}{\ln\; 2} - \frac{\left( {\Omega_{\hat{i},\hat{i}} + \eta_{\hat{i},\hat{i}}^{m}} \right)\left( {1 + {\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}}} \right)}{\gamma_{\hat{i},\hat{i}}^{m}\left( {1 - \beta_{\underset{\hat{i},\hat{i}}{{ma}\; x}}^{m} - \beta_{\underset{\hat{i},\hat{i}}{m\; i\; n}}^{m}} \right)}} \right\rbrack} \right\}}} & (34)\end{matrix}$

FIG. 6 is a process flow chart showing certain steps of an example jointoptimisation process that may be performed at step s6.

At step s50, for each tone m,

$\beta_{\underset{\hat{i},\hat{i}}{{ma}\; x}}^{m}$and η_(î,î) ^(m) are initialised to zero, and ρ_(î,î) ^(m) isinitialised to one.

At step s52, for the î channel, a value of Ω_(î,î) is computed. In thisembodiment, Ω_(î,î) is computed by using equations (33) and (29), withthe parameters initialised as in step s50, which give:

$\begin{matrix}{\Omega_{\hat{i},\hat{i}} = \frac{M}{\ln\; 2\left( {\frac{P_{T}}{\Delta_{f}} + {\sum_{m}\frac{\left( {1 + {\sum\limits_{j \neq \hat{i}}c_{\hat{i},j}^{m}}} \right)}{\gamma_{\hat{i},\hat{i}}^{m}}}} \right)}} & (35)\end{matrix}$

In this embodiment, the value of Ω_(î,î) is computed using a measuredvalue of h_(î,î).

At step s54, for each tone m, a value for η_(î,î) ^(m) is computed. Inthis embodiment, η_(m) is computed by using equation (33) and the valuefor Ω_(î,î) computed at step s52. In particular, equation (33) is setequal to p_(m) (i.e. the transmission power mask for tone m), and thenrearranged for η_(î,î) ^(m).

At step s56, for each tone m, a value for

$\beta_{\underset{j,j}{{ma}\; x}}^{m}$is computed. In this embodiment,

$\beta_{\underset{j,j}{{ma}\; x}}^{m}$is computed by plugging equation (33) into the equation log₂(1+s_(i,j)^(m)γ_(i,j) ^(m))=b_(max).

At step s58, for each tone m, a value for s_(î,î) ^(m) is computed usingequation (33) and the computed values for Ω_(î,î), η_(î,î) ^(m) and

$\beta_{\underset{j,j}{{ma}\; x}}^{m}$computed at steps s52, s54, and s56 respectively.

At step s60, for each tone m, it is determined whether or not thecalculated value of s_(î,î) ^(m) for that tone m satisfies a firstcriterion. In this embodiment, the first criterion is:log₂(1+s _(î,î) ^(m)γ_(î,î) ^(m))>0  (36)

At step s62, for each tone m for which s_(î,î) ^(m) satisfies the firstcriterion, the values for s_(î,î) ^(m) and ρ_(î,î) ^(m) of that tone aremaintained.

At step s64, for each tone m for which s_(î,î) ^(m) fails to satisfy thefirst criterion, the values for s_(î,î) ^(m) and ρ_(î,î) ^(m) of thattone are set equal to zero.

At step s66, for each tone m for which s_(î,î) ^(m) satisfies the firstcriterion, it is determined whether or not s_(î,î) ^(m) satisfies asecond criterion. In this embodiment, the second criterion is:log₂(1+s _(î,î) ^(m)γ_(î,î) ^(m))≤b _(max)  (37)

If, at step s66, it is determined that all of the s_(î,î) ^(m) valueswhich satisfy the first criterion (equation (36)) also satisfy thesecond criterion (equation (37)), then the method proceeds to step s72.Step s72 will be described in more detail later below, after adescription of method steps s68 to s70.

However, if, at step s66, it is determined that not all of the s_(î,î)^(m) values which satisfy the first criterion (equation (36)) alsosatisfy the second criterion (equation (37)), then the method proceedsto step s68.

At step s68, it is determined whether or not the current values ofs_(î,î) ^(m) satisfy a third criterion. In this embodiment, the thirdcriterion is:

$\begin{matrix}{{\Delta_{f}{\sum\limits_{m = 1}^{M}s_{\overset{\hat{}}{i},\overset{\hat{}}{i}}^{m}}} \leq P_{T}} & (38)\end{matrix}$

If, at step s68, it is determined that the third criterion (equation(38)) is satisfied, then the method proceeds to step s72. Step s72 willbe described in more detail later below, after a description of methodstep s70.

However, if at step s68, it is determined that the third criterion(equation (38)) is not satisfied, then the method proceeds to step s70.

At step s70, for each tone m, the value of Ω_(î,î) is increased. In someembodiments, for one or more of the tones m, Ω_(î,î) is increased byf(Ω_(î,î)). By way of example, f(Ω_(î,î)) may be equal to aboutΩ_(î,î)/20, Ω_(î,î)/10, or Ω_(î,î)/5. In some embodiments, Ω_(î,î) isincreased by a predetermined value. In some embodiments, for each tonem, the value of Ω_(î,î) is decreased instead of being increased.

After step s70, the method proceeds back to step s54, at which point thevalues for η_(î,î) ^(m) are updated, in turn leading to revised/updatedvalues for s_(î,î) ^(m) and ρ_(î,î) ^(m). Thus, the process of FIG. 3 isan iterative process.

Returning now the case where either 1) it is determined at step s66 thatevery tone m which did satisfy the first criterion (equation (36)) alsosatisfies the second criterion (equation (37)); or 2) it is determinedat step s18 that the third criterion (equation (38)) is satisfied, themethod proceeds to step s72.

At step s72, the iterative process of steps s54 to s70 is stopped. Also,for each î direct-channel TMP, each tone m (m=1, . . . , M) is allocatedits respective current power value s_(î,î) ^(m). It is worth noting thatsome tones may be not allocated any power, i.e. due to s_(î,î) ^(m) andρ_(î,î) ^(m) for those tones having been set to zero.

At step s74, the AN 16 transmits signals in accordance with power valuesallocated at step s72. For example, for each tone m, the AN 16 maytransmit a signal having transmission power s_(î,î) ^(m) at tone m alongî direct-channel TMP.

Thus, an example joint optimisation process that may be performed atstep s6 is provided.

In the above described method, active receiver channels (TMPs 21-23) andspare channels (TMPs 24-28) are identified. The spare channels (TMPs24-28) are “split” and controlled to provide a power contribution (e.g.a maximum power contribution) to the active receiver channels (TMPs21-23). The direct lines of the active receivers (TMPs 21-23) are thenoptimised in light of the presence of power contributions from the sparechannels (TMPs 24-28).

Advantageously, the above described method and apparatus tends toimprove allocation of supporting signals on “unused” lines in respect ofdeciding which tones to use to support which lines/receivers.

Advantageously, the above described method and apparatus tends to ensurethat multiple paths between the AN 16 and the CPE modems 51, 52, 53 arejointly optimised. Thus, the overall channel capacity tends to beoptimised.

A further advantage provided by the above described method and apparatusis that convergence to an optimal solution tends to be faster comparedto conventional processes.

The solutions provided by the above described algorithms tend to shapethe transmitted signals according to the channel response to improvedata transmission, e.g. to maximise the delivered data. Also, the abovedescribed methods and apparatus tend to take into account the maximumavailable power and the hardware limitations in transceiver design.

The above described methods and apparatus advantageously tend to provideimproved use of power budget, and also match the power budget to channelbehaviour.

Advantageously, it tends to be possible to implement the above describedmethods and apparatus for G.fast and XG.fast at all frequencies.

Advantageously, any complex functionality for implementing certainpreferred embodiments of the invention can reside solely in the accessnetwork (e.g. at an AN or DSLAM, etc.) rather than requiring any specialCustomer Premises Equipment (CPE), in certain preferred embodiments ofthe invention.

It should be noted that certain of the process steps depicted in theflowcharts of FIGS. 3 to 6 and described above may be omitted or suchprocess steps may be performed in differing order to that presentedabove and shown in FIGS. 3 to 6. Furthermore, although all the processsteps have, for convenience and ease of understanding, been depicted asdiscrete temporally-sequential steps, nevertheless some of the processsteps may in fact be performed simultaneously or at least overlapping tosome extent temporally.

It should be noted that the described embodiments are couched in termsof the downstream direction of data only (i.e. from an Access Node/DSLAMto Customer Premises Equipment (CPE) devices). However, in a practicalimplementation the “transmitter” of the above embodiments (e.g. theAccess Node) also, naturally, functions as a receiver for upstreamtransmissions from the various CPE devices (which are also therefore inpractice operating as transceivers rather than just receivers).Embodiments of the invention may operate in an entirely conventionalmanner in the upstream direction.

In the above embodiments, the DPU is connected to three user premisesvia respective direct TMP connections which connect between the ANwithin the DPU and respective CPE modems within the respective userpremises. However, in other embodiments, the DPU is connected to adifferent number of user premises (e.g. more than three) via respectiveone or more direct TMP connections. In some embodiments, the DPU isconnected to a different number of CPE modems via respective one or moredirect TMP connections. In some embodiments, one or more user premisescomprises multiple CPE modems.

In the above embodiments, there are five indirect TMP connectionsbetween the DPU and the user premises. However, in other embodiments,there is a different number of indirect TMP connections, for example,more than five.

In the above embodiments, the first criterion is:log₂(1+s _(j,j) ^(m)γ_(j,j) ^(m))>0.

However, in other embodiments, the first criterion is a differentcriterion, or is an equivalent criterion expressed in a different way.By way of example, an alternative first criterion is:log₂(1+s _(j,j) ^(m)γ_(j,j) ^(m))>ε₁

where ε₁ may be non zero. For example ε₁ may be a positive value, e.g. asmall value such as less than or equal to 10⁻², less than or equal to10⁻³, or less than or equal to 10⁻⁴. In some embodiments, ε₁ is aconstant value. In some embodiments, ε₁ is a variable that may bedependent on one or more parameters of the broadband deployment.

In the above embodiments, the second criterion is:log₂(1+s _(j,j) ^(m)γ_(j,j) ^(m))≤b _(max)

However, in other embodiments, the second criterion is a differentcriterion, or is an equivalent criterion expressed in a different way.By way of example, an alternative second criterion is:log₂(1+s _(j,j) ^(m)γ_(j,j) ^(m))≤b _(max)−ε₂

where ε₂ may be non zero. For example ε₂ may be a positive value, e.g. asmall value such as less than or equal to 10⁻², less than or equal to10⁻³, or less than or equal to 10⁻⁴. In some embodiments, ε₂ is aconstant value. In some embodiments, ε₂ is a variable that may bedependent on one or more parameters of the broadband deployment.

In the above embodiments, the third criterion is:

${\Delta_{f}{\sum\limits_{m = 1}^{M}s_{j,j}^{m}}} \leq P_{T}$

However, in other embodiments, the third criterion is a differentcriterion, or is an equivalent criterion expressed in a different way.By way of example, an alternative third criterion is:

${\Delta_{f}{\sum\limits_{m = 1}^{M}s_{j,j}^{m}}} \leq {P_{T} - ɛ_{3}}$

where ε₃ may be non zero. For example ε₃ may be a positive value, e.g. asmall value such as less than or equal to 10⁻², less than or equal to10⁻³, or less than or equal to 10⁻⁴. In some embodiments, ε₃ is aconstant value. In some embodiments, ε₃ is a variable that may bedependent on one or more parameters of the broadband deployment.

The invention claimed is:
 1. A method of transmitting data from atransmitter device to one or more receiver devices, each of the one ormore receiver devices being connected to the transmitter device via arespective wire connection, the transmitter device being operable totransmit signals onto the wire connections at one or more differenttones, the transmitter device further being operable to transmit signalsonto a further wire connection at the one or more different tones, themethod comprising: for each of the one or more tones, measuring, foreach of the wire connections, an electromagnetic coupling between thefurther wire connection and that wire connection for that tone; for eachof the one or more tones, using the electromagnetic couplingmeasurements, identifying a wire connection that most strongly receivescrosstalk interference from the further wire connection at that tone;based on the identifications, for each of the one or more tones,allocating signals transmitted on the further wire connection at thattone as supporting signals for a particular wire connection; and, forone or more of the tones: transmitting a first signal onto theparticular wire connection that has been allocated a supporting signalat that tone; and transmitting a second signal onto the further wireconnection at that tone, thereby to cause crosstalk interference in theparticular wire connection transmitting the first signal.
 2. A methodaccording to claim 1, wherein measuring the electromagnetic couplingbetween the further wire connection and that wire connection comprisesmeasuring a value of a channel transfer function between the furtherwire connection and that wire connection.
 3. A method according to claim1, wherein the further wire connection is not connected between thetransmitter device and the one or more receiver devices.
 4. A methodaccording to claim 1, wherein: the method further comprises determining,for each tone indexed by m, for each wire connection indexed by i, andfor the further wire connection indexed by j, a value of:$\mu_{i,j}^{m} = {\left( {1 - \beta_{\max\limits_{i,j}}^{m} - \beta_{\min\limits_{i,j}}^{m}} \right)\left\{ {{\log_{2}\left\lbrack \frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\max_{i,j}}^{m} - \beta_{\min\limits_{i,j}}^{m}} \right)}{\ln 2\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)} \right\rbrack} - \left\lbrack {\frac{1}{\ln 2} - \frac{\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}\left( {1 - \beta_{\max_{i,j}}^{m} - \beta_{\min_{i,j}}^{m}} \right)}} \right\rbrack} \right\}}$where: γ_(i,j) ^(m) is a channel gain in the ith wire connection causedby the further wire connection indexed by j; andΩ_(i, j), η_(i, j)^(m), β_(max_(i, j))^(m), and  β_(min_(i, j))^(m) arevariables; and the step of identifying comprises using the determinedvalues of μ_(i,j) ^(m).
 5. A method according to claim 4, wherein:$\Omega_{i,j} = \frac{M}{\ln\mspace{11mu} 2\left( {\frac{P_{T}}{\Delta_{f}} + {\sum\limits_{m}\frac{1}{\gamma_{i,j}^{m}}}} \right)}$where: M is the number of different tones; Δ_(f) is a frequency spacingbetween adjacent tones; and P_(T) is the maximum power for transmittingthe data.
 6. The method according to claim 4, wherein:$\eta_{i,j}^{m} = {\frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\max\limits_{i,j}}^{m}} \right)}{\left( {\ln\; 2} \right)\left( {1 + {p_{m}\gamma_{i,j}^{m}}} \right)} - \Omega_{i,j}}$where p_(m) is a power mask at tone m.
 7. The method according to claim4, wherein:$\beta_{\max\limits_{i,j}}^{m} = {1 - \frac{2^{b_{\max}}\left( {\ln\; 2} \right)\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}}}$where b_(max) is an upper bound for a channel capacity of the wireconnections.
 8. A method according to claim 1, wherein: the methodfurther comprises determining, for each tone indexed by m, for each wireconnection indexed by i, and for the further wire connection indexed byj, a value of:$\Gamma_{i,j}^{m} = \frac{\gamma_{i,j}^{m}}{\gamma_{i,i}^{m}}$ where:γ_(i,j) ^(m) is a channel gain in the ith wire connection caused by thefurther wire connection indexed by j at tone m; and γ_(i,j) ^(m) is achannel gain along the ith channel at tone m; and the step ofidentifying comprises using the determined values of Γ_(i,j) ^(m).
 9. Amethod according to claim 8, wherein: the method further comprisesdetermining, for each tone indexed by m, for each wire connectionindexed by i, and for the further wire connection indexed by j, a valueof:$\mu_{i,j}^{m} = {\left( {1 - \beta_{\max_{i,j}}^{m} - \beta_{\min_{i,j}}^{m}} \right)\left\{ {{\log_{2}\left\lbrack \frac{\gamma_{i,j}^{m}\left( {1 - \beta_{\max_{i,j}}^{m} - \beta_{\min\limits_{i,j}}^{m}} \right)}{\ln\; 2\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)} \right\rbrack} - \left\lbrack {\frac{1}{\ln\; 2} - \frac{\left( {\Omega_{i,j} + \eta_{i,j}^{m}} \right)}{\gamma_{i,j}^{m}\left( {1 - \beta_{\max_{i,j}}^{m} - \beta_{\min_{i,j}}^{m}} \right)}} \right\rbrack} \right\}}$where: γ_(i,j) ^(m) is a channel gain in the ith wire connection causedby the further wire connection indexed by j; andΩ_(i, j), η_(i, j)^(m), β_(max_(i, j))^(m), and  β_(min_(i, j))^(m) arevariables; the step of identifying comprises using the determined valuesof μ_(i,j) ^(m); and the step of identifying comprises, for each of theone or more tones and for the further wire connection indexed by j,determining a value of:$\hat{i} = {\arg{\max\limits_{i}\left\{ {\mu_{i,j}^{m}\bigwedge\Gamma_{i,j}^{m}} \right\}}}$wherein î denotes the identified wire connection that most stronglyreceives crosstalk interference from the further wire connection j atthat tone m.
 10. A method according to claim 1, wherein: the methodfurther comprises, for each of the one or more tones, determining apower allocation for transmitting a signal on the further wireconnection at that tone; and transmitting a second signal onto thefurther wire connection at a tone using the determined power allocation.11. A method according to claim 10, wherein determining the powerallocation comprises determining, for each of the one or more tonesindexed by m, and for the further wire connection indexed by j, a valueof:$s_{\overset{\hat{}}{i},j}^{m} = {\rho_{\overset{\hat{}}{i},j}^{m}\left\lbrack {\frac{\left( {1 - \beta_{\max\limits_{\hat{i},j}}^{m} - \beta_{\min\limits_{\hat{i},j}}^{m}} \right)}{\left( {\Omega_{\overset{\hat{}}{i},j} + \eta_{\overset{\hat{}}{i},j}^{m}} \right)\ln\; 2} - \frac{1}{\gamma_{\overset{\hat{}}{i},j}^{m}}} \right\rbrack}$where: î denotes the identified wire connection that most stronglyreceives crosstalk interference from the further wire connection j atthat tone m; s_(î,j) ^(m) is a power allocation for transmitting asupporting signal for the î wire connection on the further wireconnection j at that tone m;${\rho_{\overset{\hat{}}{i},j}^{m}\Omega_{\hat{i},j}},{\eta_{\overset{\hat{}}{i},j}^{m}\beta_{\max_{\hat{i},j}}^{m}},{{and}\mspace{14mu}\beta_{\min\limits_{\hat{i},j}}^{m}}$are variables; and γ_(i,j) ^(m) is a channel gain in the wire connectionî caused by the further wire connection indexed by j at tone m.
 12. Amethod according to claim 1, the method further comprising performing awater filling algorithm to jointly optimise a power distribution acrossthe determined allocation of supporting signals.
 13. Apparatus for usein a communication system, the communication system comprising atransmitter device and one or more receiver devices, each of the one ormore receiver devices being connected to the transmitter device via arespective wire connection, the transmitter device being operable totransmit signals onto the wire connections at one or more differenttones, the transmitter device further being operable to transmit signalsonto a further wire connection at the one or more different tones, theapparatus comprising: measurement means configured to, for each of theone or more tones, measure, for each of the wire connections, anelectromagnetic coupling between the further wire connection and thatwire connection for that tone; and one or more processors configured to:for each of the one or more tones, using the electromagnetic couplingmeasurements, identify a wire connection that most strongly receivescrosstalk interference from the further wire connection at that tone;based on the identifications, for each of the one or more tones,allocate signals transmitted on the further wire connection at that toneas supporting signals for a particular wire connection; and operate thecommunication system to, for one or more of the tones, transmit a firstsignal onto the particular wire connection that has been allocated asupporting signal at that tone, and transmit a second signal onto thefurther wire connection at that tone, thereby to cause crosstalkinterference in the particular wire connection transmitting the firstsignal.
 14. A computer program product stored on a non-transitorycomputer-readable storage medium and comprising a program or pluralityof programs arranged to, when executed by one or more processors, causethe one or more processors to operate in accordance with the method ofclaim
 1. 15. A non-transitory computer-readable medium storing one ormore programs configured to, when executed by a computer, perform themethod according to claim
 1. 16. Apparatus for use in a communicationsystem, the communication system comprising a transmitter device and oneor more receiver devices, each of the one or more receiver devices beingconnected to the transmitter device via a respective wire connection,the transmitter device being operable to transmit signals onto the wireconnections at one or more different tones, the transmitter devicefurther being operable to transmit signals onto a further wireconnection at the one or more different tones, the apparatus comprising:one or more processors configured to: for each of the one or more tones,process measurements for electromagnetic coupling for each of the wireconnections, the electromagnetic coupling being between the further wireconnection and that wire connection for that tone; for each of the oneor more tones, using the electromagnetic coupling measurements, identifya wire connection that most strongly receives crosstalk interferencefrom the further wire connection at that tone; based on theidentifications, for each of the one or more tones, allocate signalstransmitted on the further wire connection at that tone as supportingsignals for a particular wire connection; and operate the communicationsystem to, for one or more of the tones, transmit a first signal ontothe particular wire connection that has been allocated a supportingsignal at that tone, and transmit a second signal onto the further wireconnection at that tone, thereby to cause crosstalk interference in theparticular wire connection transmitting the first signal.
 17. Theapparatus according to claim 16, wherein the one or more processors isconfigured to measure the electromagnetic coupling between the furtherwire connection and that wire connection by measuring a value of achannel transfer function between the further wire connection and thatwire connection.
 18. The apparatus according to claim 16, wherein thefurther wire connection is not connected between the transmitter deviceand the one or more receiver devices.
 19. The apparatus according toclaim 16, wherein the one or more processors is further configured to:for each of the one or more tones, determine a power allocation fortransmitting a signal on the further wire connection at that tone; andtransmit a second signal onto the further wire connection at a toneusing the determined power allocation.
 20. The apparatus according toclaim 16, wherein the one or more processors is further configured toperform a water filling algorithm to jointly optimize a powerdistribution across the determined allocation of supporting signals.